The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. MDA 972-97-3-0008 awarded by the Defense Advanced Research Projects Agency (DARPA) of the U.S.
The present invention relates generally to optoelectronic devices, and, more particularly, to an optical subassembly used to convert a short wavelength laser output into a long wavelength laser output.
Light emitting diodes (LED""s), lasers, vertical cavity surface emitting lasers (VCSELs), and the like (collectively known as light emitting devices) are widely used in many applications such as communications systems, medical systems, and display systems. These light emitting devices are commonly fabricated with epitaxial materials formed on a substrate, the epitaxial materials having a p-n junction, and an active or light generation region, formed therein and typically include at least one Bragg reflector. A Bragg reflector is a fundamental building block of many light emitting devices and is used to reflect and direct the light output of a semiconductor laser.
For lasers used in communication systems, and particularly for optical communications systems, it is desirable for the laser to emit a relatively long wavelength light on the order of approximately 1.3-1.55 micrometers (xcexcm) and, in some applications, emit that light in a single spatial mode and in a single longitudinal mode. Laser emission at a single spatial mode and at a single longitudinal mode results in laser emission at single frequency. A long wavelength, single frequency output allows the laser emission to be focused into an optical fiber and to perform well in communications systems in which very high communication rates over long distances are required. Most optical fiber communication systems in use today operate either at the dispersion minimum of 1.3 xcexcm or at the loss minimum of 1.55 xcexcm.
In the past, largely because of the difficulty in obtaining low resistance, high reflectivity DBR mirrors having good long wavelength active regions in the semiconducting material system (such as indium phosphide (InP)), long wavelength VCSELs have been achieved by optically pumping a long wavelength VCSEL cavity with a short wavelength laser. The short wavelength laser was combined with the long wavelength laser using techniques such as wafer bonding, or using optical adhesive to join the two lasers. One problem with wafer bonding is that it is costly and adds at least one additional processing step. Also, there is always the possibility of damage when exposing the material to the high temperature and pressures associated with wafer bonding. Similarly, using an optical adhesive to join the two lasers increases cost and adds processing steps.
Another problem associated with joining a short wavelength laser with a long wavelength laser is that the output of the long wavelength laser also includes a short wavelength output component. That is, the prior art long wavelength laser devices emit both long wavelength light and short wavelength light. This is problematic in optical communications systems particularly over short distances because the short wavelength emission obscures the information contained in the long wavelength signal for distances which are not sufficient to filly attenuate the shorter wavelength signal.
Another problem is the thermal interaction between the wafers that have been bonded or joined by an adhesive. The heat generated during the absorption process needs to be removed as it may limit the performance of the long wavelength output. Also, the heat generated by the short wavelength laser will dramatically affect the performance of the long wavelength output.
Therefore, an unaddressed need exists in the industry for a long wavelength laser that avoids the use of wafer bonding or using optical adhesive to join two laser devices.